Gregor Mendel: The Father of Genetics PDF

Gregor Mendel: The Father of Genetics Who was he and why do we know his name today? ​ Gregor Mendel (1822–1884) was an Austrian monk who is considered the father of modern genetics. He is famous for discovering the fundamental laws of inheritance through his experiments with pea pla...

Gregor Mendel: The Father of Genetics Who was he and why do we know his name today? ​ Gregor Mendel (1822–1884) was an Austrian monk who is considered the father of modern genetics. He is famous for discovering the fundamental laws of inheritance through his experiments with pea plants. ​ Mendel’s work established the basic principles of heredity, which were later confirmed by the discovery of DNA. His work remained largely unrecognized during his lifetime, but in the early 20th century, it was rediscovered and became the foundation of genetics. Why were Pea Plants an Excellent Subject for the Study of Genetics? ​ Peas are easy to grow and have a short generation time (the time it takes from planting to harvesting seeds). ​ Peas have distinct traits (e.g., flower color, seed shape, pod color) that can be easily observed and categorized. ​ Peas can self-pollinate or be cross-pollinated by hand, which allowed Mendel to control breeding. ​ Mendel could easily control the mating of the plants, which allowed him to test specific genetic crosses and isolate traits. Mendel’s 3 Laws of Inheritance 1.​ Law of Segregation ○​ Each individual has two alleles for each gene (one from each parent), and these alleles separate during gamete formation. ○​ In a diploid organism, only one allele for each gene is passed on to the offspring. This explains the 50-50 chance of inheriting one allele or the other. 2.​ Law of Independent Assortment ○​ Genes for different traits are inherited independently of each other, meaning the inheritance of one trait does not affect the inheritance of another. ○​ This law applies only to genes located on different chromosomes or genes that are far apart on the same chromosome. 3.​ Law of Dominance ○​ When two different alleles for a trait are present, one allele may mask the expression of the other. The dominant allele is expressed over the recessive allele.  ○​ For example, if an individual has one allele for a dominant trait (e.g., brown eyes) and one for a recessive trait (e.g., blue eyes), the dominant trait will be expressed. Alleles and Representation ​ Allele: A version of a gene. For example, a gene for flower color could have different alleles such as "purple" or "white." ​ Dominant Allele: An allele that expresses its effect even if only one copy is present. It is typically represented by a capital letter (e.g., "T" for tall in pea plants). ​ Recessive Allele: An allele that expresses its effect only when two copies are present (i.e., the individual must be homozygous for the recessive trait). Represented by a lowercase letter (e.g., "t" for short in pea plants). ​ Homozygous (Purebred): An individual with two identical alleles for a gene (e.g., "TT" or "tt"). ​ Heterozygous (Hybrid): An individual with two different alleles for a gene (e.g., "Tt"). ​ Phenotype: The physical expression or appearance of a trait (e.g., tall or short plant). ​ Genotype: The genetic makeup or combination of alleles for a trait (e.g., "TT", "Tt", or "tt"). Expected Phenotypic Ratio of Mono-hybrid Crosses ​ Mono-hybrid Cross: A cross between two individuals differing in a single trait. ​ In a typical heterozygous x heterozygous mono-hybrid cross (e.g., "Tt" x "Tt"), the expected phenotypic ratio is: ○​ 3:1 ○​ 3 dominant phenotype (e.g., tall) ○​ 1 recessive phenotype (e.g., short) Expected Phenotypic Ratio of Di-hybrid Crosses ​ Di-hybrid Cross: A cross between two individuals differing in two traits. ​ In a typical heterozygous x heterozygous di-hybrid cross (e.g., "TtRr" x "TtRr"), the expected phenotypic ratio is: ○​ 9:3:3:1 ​ 9 dominant for both traits (e.g., tall and round) ​ 3 dominant for the first trait, recessive for the second (e.g., tall and wrinkled)  ​ 3 recessive for the first trait, dominant for the second (e.g., short and round) ​ 1 recessive for both traits (e.g., short and wrinkled) Codominance ​ Codominance occurs when both alleles contribute equally and visibly to the phenotype of the organism. ​ Neither allele is dominant or recessive. ​ Example: In some species of cattle, the coat color exhibits codominance. If a cow inherits a red allele and a white allele, the coat will be red and white speckled (both colors are equally visible). Incomplete Dominance ​ Incomplete dominance occurs when the phenotype of a heterozygous individual is intermediate between the two homozygous phenotypes. ​ Neither allele is fully dominant over the other, so the resulting phenotype is a mix. ​ Example: In snapdragon flowers, crossing a red flower (RR) with a white flower (WW) produces pink flowers (RW), which is a blend of both parental traits. Sex-Linked Genes (Carried on the X Chromosome) ​ Sex-linked genes are genes located on the X chromosome. ○​ X-linked traits are more commonly expressed in males because males only have one X chromosome (XY), so a recessive allele on the X will always be expressed in males. ○​ Females (XX) have two X chromosomes, so a recessive X-linked trait will only be expressed if both X chromosomes carry the allele. ​ Example: Color blindness is an X-linked recessive trait. Since males only have one X chromosome, they are more likely to be colorblind if they inherit the allele. Females would need two copies of the allele (one on each X) to express color blindness. Quick Review of Key Terms: ​ Homozygous/Purebred: Same alleles (e.g., TT or tt). ​ Heterozygous/Hybrid: Different alleles (e.g., Tt). ​ Dominant: Expressed trait when present (e.g., T). ​ Recessive: Only expressed when both alleles are present (e.g., t). ​ Allele: Different versions of a gene (e.g., T or t). ​ Trait: A characteristic that can be inherited (e.g., flower color, height). ​ Phenotype: The observable trait (e.g., tall or short). ​ Genotype: The genetic makeup (e.g., TT, Tt, or tt).